memory leak bug fix in fko_set_rand_value()

Bug fix for the following error caught by the test suite (in the [Rijndael SPA]
[client+server] random SPA port (tcp/22 ssh) test):

==24257== 17 bytes in 1 blocks are definitely lost in loss record 1 of 1
==24257==    at 0x4C2B3F8: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==24257==    by 0x4E38B9B: fko_set_rand_value (fko_rand_value.c:114)
==24257==    by 0x4E37FE0: fko_new (fko_funcs.c:75)
==24257==    by 0x10AE52: main (fwknop.c:113)